THE Math, Science and Computing School

MSC 5 – Multiplying by 2

Next easiest after multiplication by 11, is multiplication by 2. Multiplying a number by 2 is also called doubling. Doubling may be done by adding a number to itself but it is easier to apply the “times two” table that we have learned by heart.

Let us call 0, 1, 2, 3 and 4 “small digits” and 5, 6, 7, 8 and 9 “ big digits”. I am sure that there will be no problem in doubling the small digits into 0, 2, 4, 6 and 8 respectively.

Note that if we subtract 5 from a big digit, we will get a small digit. i.e.

big digit 5 minus 5 = small digit 0.

big digit 6 minus 5 = small digit 1.

big digit 7 minus 5 = small digit 2.

big digit 8 minus 5 = small digit 3.

big digit 9 minus 5 = small digit 4.

With small digits, doubling is very easy: From the left, we simply double each digit and write the answer below it.

Example 5.1:

2 4 3 1
x 2
4 8 6 2

With big digits, we modify our procedure: Double each digit starting from the left, but before we write our answer, look at the next digit to the right, if it is “big” add 1 to it. Deduct 5 from the next digit,before doubling it.

Example 5.2:

4 8
x 2
9

4 8
x 2
9 6

Doubling 4 gives 8 it before putting it down, we note that the next digit to be doubled is 8 which is big, so we add 1 first to make it 9. We then deduct 5 from 8 to make it 3 before doubling to get 6.

An important skill that can be developed by first practicing on doubling is distributedmultiplication.